While such behaviour has long been known and studied in the context of simple models of the climate system, such thresholds are now also found in the comprehensive coupled climate models currently available. (ipcc.ch)
For many biologists, slime molds provide simple models for studying cell development and mobility. (sciencemag.org)
In a very simple model of the economy, you can do only two things with your income — consume or save it. (worthwhile.typepad.com)